What is Physics?

If you are just starting out in physics, you are probably wondering just what the heck is physics, anyway? It is hard to give a concise definition of physics because it covers so much territory. It describes everyday things like levers and pulleys, but it also deals with exotic objects like black holes and subatomic particles. Webster’s dictionary defines it somewhat broadly as “The science dealing with the properties, changes, and nteraction, etc. of matter and energy”, which, if you think about it, includes all the sciences. Clearly physicists must restrict themselves to studying only some of the “properties, changes, and interaction, etc. of matter and energy”. What distinguishes physics from the other sciences is that a physicist is trying to find the most fundamental reasons why things work.

Physics is, for the most part, reductionist: We take things apart and try to understand the behavior of the smallest pieces with the hope that we will then understand how the pieces work together to produce the behavior of the original thing. A physicist is someone who needs to see the springs and gears inside a watch to understand what a watch is, while other people are happy just knowing that it gives the correct time without worrying about how it works. Not surprisingly, many physicists have a childhood history of taking apart things like radios and sewing machines just to see what was inside.

It is important to understand that this reductionist viewpoint is not always the best one. It is just one viewpoint that is sometimes useful and sometimes not. To give an extreme example, one can argue that biological processes are controlled by chemistry, and chemistry is explained by the behavior of atoms, which in turn is explained by physics. But it would be absurd to suggest that a physicist who understands atoms must therefore also understand, say, the workings of the brain or any other biological process. The physics viewpoint is only useful for analyzing situations in which we understand the connection between the “springs and gears” and the behavior of the system as a whole. We can’t do this (yet) with most of biology, and physics is not much help at all in explaining things like psychology, religion, or ethics.

Historically, physics was first concerned with describing motion. Other topics such as light, heat, and electricity became part of the investigation as the years went by, and so the concepts and terminology originally developed to describe motion was borrowed and modified to describe other phenomena. The description of motion, which we call mechanics, serves as the foundation for the rest of physics, and that is why mechanics is always the first topic covered in your sequence of physics courses.

Branches of Physics

This list is, of course, hopelessly incomplete, but it covers the main areas. No doubt I have left out a topic that is close to someone’s heart, and I apologize. The list also does not reflect the many crucial areas of overlap among the various сategories–after all, the categories are just words thought up by humans, and are not recognized by Nature. The human mind likes to divide things up into neat pigeonholes, but in reality all these categories are interdependent in such an intimate way that you cannot understand just one part without knowing something about the rest. Remember this when you start wondering “Why do I have to learn this stuff? It doesn’t apply to my major!”. Believe me, it does, although the connection might not be obvious.

Classical Physics (Before 1900)

Mechanics – the study of the motion of matter

Kinematics – description of motion (position, velocity, acceleration)

Dynamics – causes of motion (forces, torques)

Statics – matter that is not accelerating

Fluid Mechanics – the motion of liquids and gases

Vibrations, Waves, and Sound

Newtonian Gravitation – the orbits of planets

Electricity and Magnetism

Static Electricity – electric charge and forces

Currents, Electric Circuits

Resistance, Capacitance, Inductance

Electric and Magnetic Fields

Electric Potentials

Interactions with Matter

Electromagnetic Waves

Thermodynamics – the study of heat


Gas Laws

Heat Transfer

Heat Engines – converting heat into work

Entropy – law of increasing disorder

Optics – the study of light

Reflection and Refraction

Geometric, or Ray Optics – lenses, mirrors

Wave Optics – diffraction, interference

Modern Physics (After 1900)


Special Theory – new views of space and time

General Theory – curved space-time theory of gravitation

Quantum Physics

Quantum Mechanics – behavior of very small systems (like atoms)

Atomic, Nuclear, and Particle Physics – structure, interactions

Solid State Physics – properties and atomic structure of solids: crystal structure, semiconductors

Statistical Mechanics – atomic view of thermodynamics

Field Theories – modern models of subatomic particles and interactions

Cosmology – evolution and structure of the universe

Astrophysics – physics of stars, galaxies, etc.